1. Field of the Invention
The subject matter of the invention pertains to a method for analyzing the composition of a gas or gas stream in a chemical reactor. The invention pertains more particularly to the application of such a method to gases or gas streams which arise during the production of chlorosilanes and organochlorosilanes, and to a method for preparing chlorosilanes in a fluidized bed reactor.
2. Description of the Related Art
The preparation of trichlorosilane (TCS) is accomplished by reaction of metallurgical silicon (mg-Si) with HCl or by reaction of mg-Si with silicon tetrachloride (STC) and hydrogen and/or HCl. Methylchlorosilanes are prepared by reaction of mg-Si with methylchloride.
U.S. Pat. No. 4,092,446 A discloses a reactor in which a stream of hydrogen chloride is passed through a silicon bed consisting of silicon particles. The hydrogen chloride reacts with the silicon particles to form silicon tetrachloride (STC) and TCS and hydrogen.
For the economic optimization of chlorosilane syntheses in terms of the yields of the respective target products, the analysis of the gaseous reaction products is of great importance.
For example, the reaction of mg-Si with HCl leads to a product spectrum encompassing the principle product TCS (>80%), byproducts such as STC (<20%) and dichlorosilane (DCS) (<2%), plus various trace impurities. Moreover, the reaction gas also comprises H2 and unreacted HCl.
The objective of the TCS synthesis is usually for a maximum TCS yield in conjunction with complete or near-complete HCl conversion, since recovery of the HCl from the reaction offgas leads to additional costs.
US 2012189526 A1 discloses a method for preparing trichlorosilane by reacting silicon particles with tetrachlorosilane and hydrogen, and optionally with hydrogen chloride, in a fluidized bed reactor to give a trichlorosilane-containing product gas stream, the fluidized bed reactor having at least one inlet for tetrachlorosilane and hydrogen and also, optionally, for hydrogen chloride, at least one inlet for the silicon particles which form a fluidized bed with the tetrachlorosilane and hydrogen, and at least one outlet for the trichlorosilane-containing product gas stream, this outlet being preceded by at least one particle separator which selectively allows the passage only of silicon particles up to a certain maximum particle size, the characteristic feature being that silicon particles are discharged from the reactor via at least one further outlet without such a particle separator, continuously or at regular time intervals.
US 20110297884 A1 describes how a plurality of temperature measurement points positioned horizontally and vertically in the reactor are suitable for monitoring the time profile of the temperature changes in the reactor.
The reactor, however, is controlled using only one of these temperature measurement points, which lies at the upper end of the fluidized bed.
Within defined limits, a particular temperature is considered advantageous as a control variable under the selected operating conditions.
A variety of methods are described in the literature for increasing the TCS yield.
Besides compliance with particular reaction conditions, such as the quenching of the reaction gas, for example, catalysts are frequently employed in these methods. An increase in the HCl conversion is achievable, for example, through an increase in the temperature and the addition of catalysts.
A problem here is that measures for increasing the HCl conversion frequently entail a reduction in the TCS selectivity (e.g., temperature rise), or vice versa (quench, improved cooling).
In practice, therefore, it is difficult at the same time to bring about the optimum reaction conditions for TCS selectivity and for HCl conversion.
Even optimum operating conditions, once set, undergo change over the course of the reaction time. In the course of continuously operated TCS synthesis, the reactor accumulates impurities and also, possibly, catalytically active constituents, which adversely affect both TCS selectivity and HCl conversion.
It is therefore necessary to remove these impurities from the reactor regularly. This as well induces fluctuations in the TCS selectivity and in the HCl conversion.
In the reaction of metallurgical silicon (mg-Si) and HCl to give TCS (HSiCl3), hydrogen and byproducts are formed:Si+3 HCl=HSiCl3+H2+byproducts   (1)
The amount of the byproducts formed in the reaction (1) and hence the TCS selectivity, defined as mole fraction TCS/(TCS+byproducts), is influenced by a number of factors, including the catalytic effect of impurities (accompanying elements) in the mg-Si used.
It is known that impurities in mg-Si or addition of a catalyst to mg-Si may influence the selectivity of the reaction. Certain impurities have a positive influence, hence raising the selectivity. Other impurities, in contrast, have a negative influence.
US 20090060818 A1 claims a method for preparing TCS by reaction of silicon with HCl, or STC with hydrogen in the presence of silicon and catalysts. Examples of the catalyst used are Fe, Cu, Al, V, Sb or compounds thereof. Silicon and catalysts are laminated with one another and comminuted prior to the reaction. The effects of direct contact between silicon and catalyst include a distinct reduction in the yield of byproducts, thereby increasing the TCS selectivity.
U.S. Pat. No. 5,871,705 A proposes a method for preparing TCS by reaction of silicon with hydrogen chloride, comprising the contacting of at least one silane compound selected from the group consisting of dichlorosilane (DCS), monochlorosilane (MCS) and monosilane, with silicon, during or before the reaction between silicon and hydrogen chloride. Silicon is therefore contacted with a silane compound in order to remove the oxide layer on the silicon surface and hence to raise the reactivity toward HCl. Also disclosed is the conduct of the reaction between the silicon and hydrogen chloride in the presence of a catalyst with catalytic activity for the preparation of TCS from silicon and hydrogen chloride, and of an alkali metal compound. This suppresses the reaction to give STC, and therefore raises the TCS selectivity.
WO 2006031120 A1 describes a method for preparing TCS by reaction of Si with HCl gas at a temperature between 250 and 1100° C. and a pressure of 0.1-30 atm in a fluidized bed reactor, in an agitated bed reactor, or in a fixed bed reactor, wherein the Si supplied to the reactor contains less than 100 ppm of Mn. The use of mg-Si with more than 100 ppm of Mn or addition of Mn to the reactor leads to lower reactivity and TCS selectivity.
For determining the yields of the TCS synthesis, the usual approach to date has been to condense and then analyze the chlorosilanes present in the reactor offgas. This type of offline analysis, e.g., offline gas chromatography (GC), takes up a fair amount of time and is prone to error on account of the differences in condensability of the sample constituents. With this methodology, moreover, it is not possible to determine the fractions of H2, N2 and HCl.
WO 2010135105 A1 discloses a method for analyzing gases in a method for preparing high-purity silicon, in which a gas or gas mixture comprising one or more of the gases in the H2, SiH4, H3SiCl, HSiCl3, H2SiCl2, HCl, SiCl4 and N2 group is exposed to the radiation of a Raman spectrometer in order to give a Raman signal for each of the gases present and to analyze these signals in order to ascertain the presence and concentration of any one of the gases present. The possibility for simultaneous measurement of chlorosilanes, and also H2, N2 and HCl, by means of Raman spectroscopy is viewed as an advantage for rapid intervention in the processes for the deposition of polysilicon and for the conversion of STC to TCS.
WO 2011026670 A2 discloses a method for controlling a plant for the preparation of polycrystalline silicon, in which the plant comprises at least one reactor having at least one feed line and one offtake line for a gas mixture, the method characterized by the following steps:                samples for measurement are taken from the feed line and the offtake line of each reactor;        the samples for measurement that are taken are supplied, each via a line, to at least one gas chromatograph;        measurement values obtained using the gas chromatograph and relating to the composition of the measurement samples supplied are used to derive control signals; and        the control signals obtained, by means of a controlling and regulating unit, are used to adjust, via actuating elements, a multiplicity of parameters of the at least one reactor in such a way that the efficiency of the plant is guided automatically to a production optimum.        
It has emerged that the methods described in WO 2010135105 A1 and in WO 2011026670 A2 are disadvantageous when used to analyze gas streams from silane syntheses. It has been observed that particles are deposited on the measuring cells or on optical components. Cleaning the measuring equipment has not been able to provide a remedy, since the particles are evidently abrasive particles which may damage the components.
It has also been ascertained that there are alterations in the background of the spectra and there is a continuous loss in intensity of the signals, and that these factors make analytical evaluation more difficult or else impossible. In the case of gas chromatography, there were instances of blocking in lines and valves, and also of unwanted reactions with the separation medium in the column of the gas chromatograph.
The problems which have been observed in the prior art, such as depositions on or damage to components, losses in intensity of the spectra, instances of blocking or reaction with separation medium, do not occur in the method of the invention. The inventors assume that the problems in the prior art are caused by depositions of AlCl3 and/or dust particles. The resublimation of AlCl3 on colder points of the measuring apparatus (e.g., of the optical components) would appear to lead to a continuous loss of intensity of the signals and to further alterations in the Raman spectrum, which would no longer permit reliable evaluation.
The problems described gave rise to the objective of the invention.